The present invention relates generally to systems for monitoring soliton transmission performance within optical systems, and specifically to systems for monitoring transmission performance of soliton pulses within an optical transmission system by monitoring dispersive-wave energy relative to energy within the soliton pulses.
In optical transmission systems, certain optical effects are known to degrade the quality of transmission along standard transmission optical fiber in certain circumstances. Group-velocity dispersion is one of these optical effects and provides a limitation to quality transmission of optical signals across long distances. Group-velocity dispersion typically broadens an optical pulse during its transmission across long distances, which may lead to dispersion of the optical energy outside a time slot assigned for the pulse. Consequently, a trend in optical communications is toward the use of soliton pulses that maintain their pulse width over longer distances by balancing the effects of group-velocity dispersion with the nonlinear phenomenon of self-phase modulation. In this manner, the combined effect of group-velocity dispersion and self phase modulation effectively cancel each other when using soliton pulses. Those skilled in the art will be familiar with soliton pulses within optical transmission systems.
Additionally, one of the known advantages of using a soliton-pulse within an optical transmission system is that the soliton-pulse is robust to small perturbations as it propagates down an optical fiber. In other words, small temporal distortions, non-optimal group-velocity dispersion and small variations in power or pulse shape usually will not affect the stability of a soliton-pulse as it propagates down an optical fiber. Thus, it is theoretically possible for the soliton-pulse to travel an indefinite distance without degrading or changing its pulse shape.
However, there are some problems that may limit the useful transmission distance of soliton pulses within an optical transmission system. A known condition when using soliton pulses is that any non-solitonic pulse energy is essentially thrown out of the soliton-pulse and into what is conventionally called a dispersive wave. The dispersive-wave essentially operates as a photon "bin" for excess pulse energy outside a conventional soliton solution to the nonlinear Schrodinger wave equation. Those skilled in the art will understand that the nonlinear Schrodinger wave equation is a differential equation in the technical field of quantum mechanics governing optical fiber transmission of waves. As long as the dispersive-wave energy is not too large, the soliton pulse propagation may remain substantially undistorted and without significant soliton pulse to dispersive wave interaction. Thus, while the soliton-pulse propagates down the fiber, the energy level in a dispersive-wave may build up or increase due to the non-solitonic dispersive energy from any degradation of the soliton-pulse.
One problem that may be encountered when using soliton pulses is determining how to gauge or monitor the performance of transmitted soliton pulses as the pulses begin to degrade. Applicant has observed that when a series of soliton pulses (also known as a soliton-pulse stream) is transmitted or injected within an optical fiber, a dispersive-wave of non-solitonic energy appears between adjacent soliton pulses. In this manner, some dispersive-waves are essentially trapped between adjacent soliton pulses. For isolated soliton pulses, the dispersive-wave energy increases energy in the adjacent time slot representing a logical zero. In other words, the dispersive-wave energy reduces the extinction ratio (i.e., ratio of average optical power in a logical one to average optical power in a logical zero) when isolated soliton pulses are encountered. Furthermore, Applicant has discovered that conventional soliton transmission systems do not actively monitor the build up of such dispersive energy.
Patents and publications have described the existence of dispersive waves, their interaction with soliton pulses and how to compensate for dispersive degradations of soliton pulses. For example, U.S. Pat. No. 5,767,998 discloses a wavelength-division multiplexed optical transmission system utilizing optical soliton pulses. The '998 patent discloses optical amplifiers. When inserted into an optical fiber at predetermined intervals, the optical amplifiers compensate for loss in the fiber. Additionally, the '998 patent discloses eliminating any dispersive waves generated by soliton pulses and eliminating disturbance caused by soliton collisions by varying the fiber's dispersion characteristic in regions at and between the optical amplifiers.
U.S. Pat. No. 5,471,333 discloses another optical transmission system utilizing optical soliton pulses. The '333 patent discloses compensating for broadening of the soliton pulses by inserting an optical amplifier into an optical fiber and returning the soliton-pulse to its initial value. Furthermore, the '333 patent discloses controlling wavelength-dispersion values of the fiber in particular sections by using sections of fiber having dispersion values alternating larger and smaller than an average dispersion value meeting a predetermined soliton condition.
In an article authored by Pierluigi Franco, Michele Midrio, Marco Romagnoli, and Stefan Wabnitz entitled "Relaxation of Guiding Center Solitons in Optical Fibers" and published in Optics Letters, Vol. 21, No. 17 published on Sep. 1, 1996 (hereinafter "the Franco article"), the authors describe a resonance between optical amplifiers periodically located along an optical fiber link and soliton pulses leading to the generation of dispersive-wave energy. The dispersive-wave energy is disclosed in the Franco article to appear as sidebands to the soliton pulses. The Franco article further discloses how soliton perturbation theory still applies when strongly perturbed solitons appear in periodically amplified optical-fiber links.
In an article by R. J. Essiambre and G. P. Agrawal entitled "Control of Soliton-Soliton and Soliton-Dispersive Wave Interactions in High Bit-Rate Communications Systems" published by IEEE Electronic Letters, Vol. #31, No. 17 published on Aug. 17, 1995 (hereinafter "the Essiambre article"), the authors disclose that system performance of a soliton-based lightwave system is affected by the interaction between solitons and dispersive waves. The Essiambre article discloses that numerical simulations for high bit-rate (80 Gbit/s) soliton communication systems show that limiting factors include the growth of dispersive waves that interact with a soliton train of pulses. The Essiambre article also discloses that transmission distance can be increased by either inserting fast saturable absorbers or using synchronous modulators to control the soliton-to-dispersive-wave interaction.